Unlike traditional temperature sensors that utilize offchip components, on-chip CMOS (complementary metal-oxide-semiconductor) temperature sensors are well-known for their benefits of low production cost and ability to interface easily with other electronic circuits. On-chip temperature sensors with small area and low power consumption are normally employed in thermal management applications in order to monitor system reliability and performance as a result of temperature variation.
One popular use of embedded temperature sensors in very-large-scale integration (VLSI) implementations is to monitor excess on-chip heat dissipation resulting from higher and higher levels of integration. On the other hand, the emergence of radio frequency identification (RFID) and wireless sensor network (WSN) applications has given rise to the development of embedded temperature sensors for wireless monitoring systems. For deployment in such applications, power consumption, rather than sensing range and accuracy requirements, is a primary consideration, especially for passive RFID tags. This is because passive RFID tags harvest power through rectification of the incoming RF signal, and the total energy available is limited. Typically, the total current budget for an entire RFID tag may be as low as a few μAs. Thus, the addition of an embedded sensor increases the tag loading, and the tag operating distance will be reduced as a result.
On-chip temperature sensing is conventionally accomplished using BJT devices and digitized using analog-to-digital converters (ADCs). These sensors generally achieve good accuracy but are associated with penalties of increased circuit complexity and chip area. The corresponding power consumption is usually in the μW range and is therefore not suitable for passive RFID tag applications.
The use of delay generated by inverter chains for temperature sensing with time-to-digital converters (TDCs) for digitizing temperature modulated pulse-width has also been implemented. These time domain sensors, though having increased inaccuracy, usually outperform sensors utilizing ADCs in terms of power consumption and area.
In a few recent works, various passive RFID tags embedded with temperature sensors with relatively low power consumptions have been reported. In K. Opasjumruskit et al, “Self-powered Wireless Temperature Sensor Exploit RFID Technology,” IEEE Pervasive Computing, vol. 5, issue 1, pp. 54-61, January-March 2006, an on-chip temperature sensor based on BJT architecture with sigma-delta ADC is demonstrated. The tag consumes 2.4 μW and 12 μW (assuming a 1.2V supply) for read and temperature measurement operations, respectively, while achieving an inaccuracy of −1.8° C./+2.2° C. from 0 to 100° C. after calibration at 40° C. In this case, the sensor requires much more power than the rest of the tag, and thus the addition of the sensor can significantly affect normal tag operation.
In N. Cho et al, “A 5.1 uW UHF RFID Tag Chip integrated with Sensors for Wireless Environmental Monitoring,” European Solid-State Circuits Conference, pp. 279-282, September 2005, temperature is measured by charging an integrating capacitor up to the temperature-dependent diode voltage VBE through a reference current. The time required is then digitized using a reference clock. The sensor dissipates a total current of 1.6 μW, with a resolution of 0.8° C. and an inaccuracy of ±2.4° C. within the −10 to 80° C. sensing range. Even though the sensor only consumes 1.6 μW, it is still a large amount when compared with the reported tag total power consumption of 5.1 μW.
In Y. Lin et al., “An Ultra Low Power 1V, 220 nW Temperature Sensor for Passive Wireless Applications,” IEEE Custom Integrated Circuits Conference, pp. 507-510, September 2008, a standalone temperature sensor with only 220 nW power consumption is reported. In this article, both the temperature-dependent and the reference currents are converted to frequency signals. The sensor exhibits a temperature inaccuracy of −1.61+3° C. from 0° C. to 100° C., while occupying an area of 0.05mm2. However, even though sub-μW CMOS temperature sensing is demonstrated, a high supply voltage of 1V is still required for proper sensor operation.
Even with these improvements, existing solutions for embedded ultra-low power temperature sensors in passive RFID tags still consume too much power relative to the limited power available, which greatly reduces the normal tag reading range. As an example, an increase in tag power consumption by 30% through activating the sensor—as in the case of N. Cho et al., “A 5.1 uW UHF RFID Tag Chip integrated with Sensors for Wireless Environmental Monitoring”—can reduce the tag operating distance by almost 20% based on the Friis Transmission Equation (assuming other parameters to be constant).
Embedded temperature sensors should ideally consume power in the sub-microwatt range, so as not to affect the overall tag operation. Based on recent developments in RFID tag technology, overall tag power (and hence the amount of power available to be allocated to a temperature sensor) will continue to decrease in the future, accompanied by increases in tag operating distance, making ultra-low power temperature sensing even more important in future designs. While minimizing power consumption is an object underlying certain implementations of the invention, it will be appreciated that the invention is not limited to systems that solve the problems noted herein. Moreover, it will be appreciated that the inventors have created the above body of information for the convenience of the reader; the foregoing is a discussion of problems discovered and/or appreciated by the inventors, and is not an attempt to review or catalog the prior art.